The present invention relates to bone fracture compression plates, and in particular to non-stress-shielding plates.
Compression plates typically are attached to the bone by screws to transfer the load from the bone to the plate through the screws and force the broken ends of the bone together. Refinements to this basic device have been made. Other typical fracture fixation compression devices are disclosed in U.S. Pat. Nos. 4,513,744; 3,779,240; and 3,552,389.
When a conventional bone fracture compression plate made of high modulus 316L stainless steel (200 GPa; 30.times.10.sup.6 psi) relative to cortical bone (15 GPa) is used, this results in stress-shielding the bone under the plate and is believed to cause an osteoporosis that weakens the bone under the plate. Relieving the bone from carrying a load over an extended period of time is believed to contribute to the development of this type of osteoporosis, also known as osteopenia. When the plate and screws are removed from the healed bone, the bone may refracture due to the weakening which resulted from the development of osteoporosis or osteopenia.
Attempts to solve this problem have included the fabrication of the fracture plates from materials that are less rigid than 316L stainless steel. For example, titanium alloys, composites, and resorbable materials have been tried. However, each of these materials presents additional problems.
Fracture plates require screw holes which tend to acquire nicks and notches from contact with the screws. Titanium alloys require heat treatment for strengthening, but such treatment lowers the shear strength, thus rendering the plate particularly sensitive to nicking and notching. The nicks and notches become sites for the development of undesirable corrosion.
An investigation using graphite fiber reinforced polymethylmethacrylate (GFMM) as the plate material is reported in Woo et al, "A Comparison of Cortical Bone Atrophy Secondary to Fixation with Plates and Large Differences in Bending Stiffness," J. Bone Jt. Surg., Vol. 58A, pp. 190-195 (1976). Composites present the usual problems of debonding, low fatigue limit, etc.
Bioresorbable bone plates have been investigated in Christal et al, "In vivo fate of bioresorbable bone plates in long-lasting poly (L-lactic acid)," p. 279, Trans. Second World Congress on Biomaterials, Apr. 27-May 1, 1984, Washington, D.C. The main problems associated with resorbable bone plates made of polyglycolic acid (PGA) or polylactic acid (PLA) polymers are the release of large amounts of the polymer residues into the body. It is not yet proven whether these polymer residues produce any side effects. However, the precise control of the dissolution rate of these polymers over the entire plate is difficult, if not impossible, due to many factors. Moreover, the screws cannot be made from these materials because these materials lack the requisite torsional strength. Hence, conventional metal screws must be used, and a secondary operation to remove the screws cannot be avoided, even when the plates are made of resorbable materials.
Other attempts at solving the stress-shielding problem include the use of non-conventional types of fracture plates.
A distinction is made between compression devices disclosed above which are removed at a certain point during the healing process, and non-compression "conventional" fixation devices. One difference between a compression plate and a non-compression fixation device is the shape of the holes used in securing the respective devices to the bone. Non-compression fixation devices typically use holes having a generally frustoconical profile, while compression plates use holes having a more spherical profile. This is because the former is intended to remain fixed, and the latter is intended to allow the screws to be offset or directed at an angle into the bone. For example, in U.S. Pat. No. 3,596,656 to Kaute, a non-compression fixation device comprises a frustoconical shape bore and a similarly shaped washer which can be formed of high density, high molecular weight polyethylene. The washer is fitted into the countersunk portion of each plate bore to avoid corrosion and electrical effects caused by contact between the bore surface and the screw inserted through the bore into the bone. The washer can include a coaxial straight-sided tubular neck part that terminates spaced from the backface of the plate and that has an outer diameter smaller than the bore diameter and an inner diameter larger than the diameter of the screw shank.